Various methods are known for the selective electrochemical etching of metals. According to these methods, the workpiece whose surface is to be etched is generally made the anode and contacted with the electrolyte into which the cathode is immersed. A surface can be selectively etched by limiting the contact of the electrolyte to only those areas of the surface that are to be etched. This can be achieved by covering the surface areas which are not to be etched with a suitable masking material, or by limiting the electrolyte contact to only the surface areas to be etched.
In multilayer circuit board fabrication, a copper foil is laminated onto a dielectric layer, which is laminated onto an inner printed circuit layer. The copper foil is etched to produce a roughened surface. This roughened surface is then protected from subsequent processing steps by applying an adhesive tape, such as polyester tape, over the copper layer. In order to make electrical connection to the inner layers, holes are drilled through the multiple layers and then cleaned chemically and/or mechanically. The adhesive tape is then removed from the roughened copper surface. It is often necessary to remove the adhesive of the tape from the copper surface by using halogenated hydrocarbon solvents. Once the tape and adhesive are removed, the surface of the multilayer circuit and the through holes are electrolessly plated to form an electrical contact to the inner layers.
With the method of the present invention, it is possible to first drill the through holes in the multilayer circuit board, and then etch the surface of the copper foil layer without etching or damaging the copper of the inner layers which is exposed to the etching solution in the through holes. There is no need, therefore, to use polyester tape to protect the copper foil since the copper foil is etched after the through holes have been drilled.
It is an object of the present invention to eliminate the need to use polyester tape in the fabrication of multilayer circuit boards to reduce the steps required, and therefore, the cost of producing multilayer circuit boards. In addition, because the use of polyester tape masking has significant environmental disadvantages due to the halogenated solvents required for tape adhesive removal, it is a further object to eliminate the use of halogenated solvents.